The invention concerns an assembly of an end-fitting and a flexible pressure pipe which is of a non-bonded structure comprising a number of layers including at least one layer having a number of helically wound flat metallic tensile armour wires with end parts which, in the assembled condition, are embedded in an anchor consisting of a casting material, e.g. a polymer such as epoxy, which is injected into a cavity formed in the end-fitting.
Flexible pressure pipes are used in a variety of applications, including water supply lines, sewage lines and lines for transporting chemicals such as liquid ammonia and phosphoric acid and also high pressure offshore flexible pipes for the oil and gas industry.
The tensile armour layer of the flexible pressure pipe and the joint between this layer and the end-fitting provides the majority of the resistance to axial tensile loads acting on the assembly of the end-fitting and the pipe. Such loads can for many applications be very high.
In a conventional end-fitting, the joint between the tensile armour layer and the end-fitting is obtained by providing the end part of each flat tensile armour wire with a bent hook-like portion prior to embedding the end part in the casting material.
Such an end-fitting and a pressure pipe having wire parts for anchoring the wires in the casting material are known from DE 1 032 987. These known wire parts are bent 180xc2x0.
The resisting moment of each flat wire is, however, relatively small and, moreover, as there is no or only a little friction between the casting material and the wires these will tend to be drawn out of the casting material while straightening out the bend when acted upon by an axial tensile load which is smaller than the tensile armour layer itself is able to withstand. In those situations in which the flexible pipes are to be used for high pressures and/or are descending over a great length it may be difficult to provide the assembly with sufficient strength to withstand the great axial tensile loads acting thereon.
To overcome the above mentioned problems, in some previous embodiments the ends of the tensile armour wires have been welded to a part of the end-fitting. In this way the strength of the joint between the layer and the end-fitting has been increased, but at the expense of a rather high production cost because welding is a very time-consuming process which must be carried out over a long period of time to ensure that the polymer layers are not harmed by heat from the welding.
The object of the invention is to provide an assembly of the type mentioned in the opening paragraph in which the joint between the layer of helically wound flat metallic tensile armour wires and the end-fitting is a simple and cheap structure which provides a greater resistance to axial tensile loads acting on the assembly than has been known before.
This is achieved in that at least some of the flat wire end parts have at least one twist, turning generally around the centreline of the wire. This twist will lock the end part of the wire firmly in the casting material because the twist must be straightened out before the end part can be drawn out of the casting material. To straighten out the twist, when the wire is acted upon by a large tensile force, the twist must be acted upon by a sufficiently high reactive torque from the casting material and such high torque is difficult or nearly impossible to provide because the torque acts with a very short moment arm upon the twist.
The joint between the end parts of the tensile armour wires and the end-fitting can be provided with a superior strength when each of the end parts have at least two successive twists turning in opposite directions to each other, such that the joint between the end part and the casting material is secured against loosening by preventing a screwing movement of the twists.
An optimal joint is furthermore achieved when each twist extends over a length of between one and three times the width of the cross-section of the flat wire, because the material of the flat wire is not then overloaded when being twisted and the flat surface of the wire in the twist is at the same time turned sufficiently sharply across the axis of the wire so as to be firmly anchored in the casting material.
To counteract straightening of the twists which would be sufficient to allow them to be drawn out of the casting material, it is advantageous if the twists are turned more than 20xc2x0 and especially about 90xc2x0.
As mentioned above, each twist is acted upon by a high reactive torque from the casting material when the wire is under a heavy tensile load. As the moment arm of the torque is short, the torque itself will consequently generate high compressive loads upon the casting material tending to crush it. To avoid a dangerous concentration of compressive loads on the casting material, two successive twists on the same wire can advantageously be separated by a substantially straight length of wire.
A flexible pressure pipe typically has two counter wound layers of helically wound flat metallic tensile armour wires. In this case a concentration of compressive loads on the casting material can be avoided when the wire end parts of one of said layers form a different angle with the axis of the end-fitting to the wire end parts of the other layer.
A compact construction with a small diameter is obtained when the centreline of the flat wire end parts extends along a generally straight line.
The invention also concerns a method for assembling an end-fitting and a flexible pressure pipe of the above named type and which comprises removing the layers surrounding the wires at the end of the pipe, forming at least one twist in at least some of the flat wire end parts by turning the wires generally around their centrelines, placing the end parts into a cavity formed in the end-fitting, and injecting a casting material, e.g. a polymer such as an epoxy, in said cavity. This method can be carried out in situ.
Two successive twists can advantageously be formed at the same time on a wire end part by engaging the end part with two jaws of a first tool part, engaging the end part with a jaw of a second tool part between said two jaws of the first tool part and turning the two tool parts in relation to each other generally around the centreline of the wire.